The present invention relates generally to antennas, and more particularly to ultra-wideband antennas. The present invention also relates generally to antennas which incorporate a ground plane, to monopole antennas, and to antennas driven with an unbalanced power source.
A typical radio-communications antenna, such as an AM, FM or television antenna, is designed to operate efficiently for reception and/or transmission over a range of frequencies which is small relative to the central frequency of the range. Much theoretical and empirical research has been devoted to the design of such antennas. Less common are wideband antennas where the range of frequencies over which the antenna operates is not small in relation to the central frequency transmitted. Non-sinusoidal spread-spectrum radio communications (i.e., communications where pulse sequences are transceived) require ultra-wideband antennas since the frequency components of a pulse with time width xcex4t extend all the way from zero frequency to frequencies on the order of 1/xcex4t. Therefore, the transmission of a 1 nanosecond pulse requires an antenna with a frequency response that extends all the way from 0 Hz to around 1 GHz.
Ultra-wideband antennas are difficult to design because numerous approximations used in the design of standard antennas do not hold, particularly if the frequency range must extend into the gigahertz. For instance, skin-depth effects become important, emissions from various portions of the antenna interact with current flows in other portions of the antenna, the velocity of current flow within the antenna must explicitly be taken into account, etc.
A dipole large-current radiator (DLCR) as taught by the prior art is shown in FIG. 1. (See Henning Harmuth and Shao Ding-Rong, xe2x80x9cAntennas for Nonsinusoidal Waves. I. Radiators,xe2x80x9d IEEE Transactions on Electromagnetic Compatibility, Vol. EMC-25, No. 1, February 1983.) The DLCR (100) consists of a main radiator (105), side leads (117a) and (117b), rear leads (120a) and (120b), power leads (121a) and (121b), and a power source (140). Side lead (117a) is attached to the horizontally-oriented, main radiator (105) at a first end (106a) and extends downwards therefrom, and consists of an upper, flared section (115a) and a lower, thin section (116a). (Terms such as xe2x80x9chorizontal,xe2x80x9d xe2x80x9cvertical,xe2x80x9d xe2x80x9cleft,xe2x80x9d xe2x80x9cright,xe2x80x9d xe2x80x9cabove,xe2x80x9d and xe2x80x9cbelowxe2x80x9d are used in the claims and in the descriptions of the antennas in the present specification in reference to the accompanying figures for ease of explanation to describe relative positions, and are not intended to imply that the antennas can only be oriented in the directions shown in the figures.) Similarly, side lead (117b) is attached to the main radiator (105) at the other end (106b) and extends downwards therefrom, and consists of an upper, flared section (115b) and a lower, thin section (116b). The lower ends of the side leads (117a) and (117b) are connected to rear leads (120a) and (120b) which extend therefrom in the xe2x88x92x and +x directions, respectively. The inside ends of the rear leads (120a) and (120b) connect to power supply leads (121a) and (121b), respectively, which extend vertically downwards.
The power supply leads (121) are connected to a balanced power supply (140), i.e., a power supply where the voltage at one terminal is of equal magnitude but opposite polarity from the voltage at the other terminal. (In the present specification, a reference numeral which has a three-digit number section and is not appended by a letter will be used to refer generically to pairs of elements whose references numerals have the same three-digit number section and end with a letter.) The DLCR (100) of FIG. 1 is thus considered a xe2x80x9cdipolexe2x80x9d antenna because it is symmetric about the dividing line (104) at the mid-point of the current flow, and centrally powered by a balanced current source, so the current in the antenna (100) is symmetric about the dividing line (104). For instance, as a current propagates from the first edge (106a) to the middle of the main radiator (105), a current of the same magnitude and in the same direction will propagate from the main radiator (105) into the opposite edge (106b). Similarly, the rear leads (120) form a second radiating dipole. To some extentxe2x80x94the extent being determined by the degree to which the currents in the rear leads (120) and the main radiator (105) are out of phasexe2x80x94the combination of the main radiator (105) and the rear leads (120) will function as a quadrupole radiator, and thus have limited efficiency over much of the solid angle around the DLCR (100). However, because much of the radiation emitted upwards from the rear leads (120) is blocked (or xe2x80x98shieldedxe2x80x99) by the main radiator (105), in the +z direction the DLCR (100) will function more like a dipole radiator. (Because the side leads (117) are parallel, have the same size and shape, and are powered by the balanced, low-impedance power source (140), signals radiated from them (117) will tend to have equal magnitude but opposite polarities, and will substantially cancel. More particularly, radiation from the side leads (117) will fall off with distance r faster than 1/r2.) The DLCR (100) is considered a xe2x80x9clarge-currentxe2x80x9d antenna because it is a low-impedance closed circuit spanning the output of the power supply (140). Since the far-field emissions about a conductor is proportional to the first time-derivative of the current distribution, the advantage of a large-current antenna is that large current changes, and therefore large emissions, can be produced.
Harmuth also teaches putting a wide radiation shield (not shown) directly under the main radiator (105), i.e., between the main radiator (105) and the rear leads (120), to absorb radiation from the rear leads (120). This allows the antenna (100) to function as a dipole radiator over a much wider range of solid angle. Although the above-referenced paper by Harmuth calculates the transmission characteristics of this antenna (100), construction of such an antenna (100) is problematic since: radiation shields, such as ferrite absorbers, generally do not have a permeability exceeding 10 gauss/oersted at frequencies of gigahertz; and if an absorber with a permeability on the order of 1000 gauss/oersted could be constructed, it would be bulky, weighty and expensive.
A further limitation of the DLCR (100) of FIG. 1 is that it cannot be used in applications where the equipment must have a ground plane in the vicinity, because of the substantial distortions caused by the radiation generated by image currents. Printed circuit boards have a ground plane, and, generally, portable, battery-powered transceivers use circuit-board circuitry and an unbalanced power source with one terminal connected to the ground plane. Although an ultra-wideband balun might be used to transform the unbalanced antenna signal to a balanced antenna signal, baluns are large and expensive.
Therefore, it is an object of the present invention to provide an ultra-wideband antenna, i.e., an antenna which can efficiently and accurately transceive pulses, particularly pulses on the order of 1 ns in length.
It is another object of the present invention to provide a large-current antenna, i.e., closed-loop, low-impedance antenna.
More particularly, it is an object of the present invention to provide a large-current and/or ultra-wideband antenna that performs well over a wide range of solid angle.
It is another object of the present invention to provide a large-current and/or ultra-wideband antenna that operates without use of an absorber.
It is another object of the present invention to provide a large-current and/or ultra-wideband antenna which incorporates a current-imaging conductor, such as a finite-size ground plane.
It is another object of the present invention to provide a large-current and/or ultra-wideband antenna which is powered by an unbalanced current source.
Additional objects and advantages of the invention will be set forth in the description which follows, and will be apparent from the description or may be learned from the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.